This invention relates to integrated circuits with differential signals, and more particularly to a power-down signal encoded in a differential signal.
One technique for increasing the signaling speed of integrated circuits (IC""s) is to use differential rather than single signals. A differential pair of signals has two physical lines that move in opposite directions when changing state: one differential signal line is driven to a higher voltage while the other differential line is driven to a lower voltage. At steady-state, one of the differential signal lines is in a high state while the other differential line is in a low state. The difference in voltage between the high and low states can be only a few hundred milli-volts, minimizing voltage swing, capacitive charging delays, and overall signal propagation delay.
Many modern electronic systems are portable, running on limited battery power. Other systems must limit power consumption to reduce cooling requirements or electric power costs. Often some parts of a system are not in continuous use and can be powered down when idle.
FIG. 1 highlights power-down of differential signaling between two chips. Transmitter chip 14 contains differential transmitter 10, which drives differential output lines V+, Vxe2x88x92 to opposite states. Differential receiver 12 in receiver chip 16 senses a small voltage difference between differential input lines V+, Vxe2x88x92 and outputs a single signal to logic inside receiver chip 16. This signal can be a clock signal generated by transmitter chip 14.
Transmitter chip 14 may contain control logic that determines when the functions performed by receiver chip 16 are no longer needed. Transmitter chip 14 activates power-down signal PWR-DN which connects to an enable input of receiver chip 16. Activating PWR-DN causes receiver chip 16 to enter a low-power state.
Sometimes the connection between chips 14, 16 can be broken, such as when a cable is unplugged. FIG. 2 shows a fail-safe circuit that blocks data in a receiver when a differential connection is broken. The differential lines driven by differential driver 10 in transmitter chip 14 are disconnected from receiver 12, perhaps by a disconnected cable. Pull-up resistors 24, 26 near receiver chip 16xe2x80x2 pull the differential inputs high when the cable is disconnected. NAND gate 22 detects when both of the differential inputs are in the high state, which does not occur during normal operation.
NAND gate 22 drives the fail-safe signal FSB low when the Hxe2x80x94H condition is detected, causing AND gate 20 to output a low, regardless of the data condition sensed by receiver 12. Thus downstream logic in chip 16xe2x80x2 is protected from indeterminate data and metastability by the fail-safe circuit.
Oftentimes the number of available pins on an integrated circuit chip is limited. Pin functions can be combined to save pins, such as by using an encoded clock using Manchester Encoding, or by using illegal conditions (states of one or more pins) to signal a seldom-used mode.
FIG. 3 shows a power-down mode encoded by an illegal high-high state of differential lines. Transmitter chip 14xe2x80x2 generates an internal power-down signal PDN when control logic determines that receiver chip 16 should power down. This PDN signal causes modified differential transmitter 10xe2x80x2 to ignore its data input and instead drive both differential outputs to a high state. Normally one or the other differential line is driven low, so the high-high condition is abnormal or illegal.
NAND gate 22 in receiver chip 16xe2x80x3 detects when both differential lines are in the high state, and activates the internal power-down signal PDNB by driving it low. This internal power-down signal then powers down logic in receiver chip 16xe2x80x3. This may include logic downstream of receiver 12, so that an indeterminate state of the output from receiver 12 does not propagate.
Rather than signaling the power-down condition using the high-high state, the low-low state can be used. However, logic in receiver chip 16xe2x80x3 must be modified to detect this low-low condition rather than the high-high condition. A disadvantage of this technique is that transmitter 10 in transmitter chip 14 must be modified to generate the high-high or low-low condition. Both chips 14, 16 must be modified and designed to operate with one another. A standard differential transmitter cannot be used, since a standard differential transmitter does not generate the illegal Hxe2x80x94H or Lxe2x80x94L state.
Such a modification of the differential transmitter may not always be possible, such as when the differential transmitter is on a personal computer (PC) driving a clock to a memory module. While the differential receiver on the memory module could be modified as new versions of memory modules are designed, the differential transmitter cannot be modified for pre-installed computers.
FIG. 4A shows a standard differential transmitter with an enable. Standard transmitter chip 28 includes differential transmitter 30 that drives a pair of differential lines to receiver chip 16xe2x80x3. NAND gate 22 can detect the high-high condition and activate a power-down mode in receiver chip 16xe2x80x3. However, differential transmitter 30 cannot drive both differential lines high.
When the enable input to differential transmitter 30 is de-activated, differential transmitter 30 stops driving both differential lines and enters a high-impedance (high-Z) state. This high-Z state is not the same as a high state.
FIG. 4B is a waveform of a standard differential transmitter entering the high-Z state. When enable ENA is high, differential transmitter 30 drives differential lines V+, Vxe2x88x92 to opposite states as the data input to differential transmitter 30 toggles. When ENA goes low, differential transmitter 30 is disabled and stops driving the differential lines. The differential lines then float.
If the differential lines were completely isolated, the voltage would remain in the prior state. However, capacitive coupling from other signals and leakage can upset the voltages on the differential lines, causing them to drift. Leakage between the two differential lines can cause the voltages to equalize over time. Any terminating resistors can also alter the voltages on the differential lines.
Since NAND gate 22 in receiver chip 16xe2x80x3 only detects dual high voltages, the floating differential lines do not necessarily trigger the power-down state. Indeed, as other signals capacitivly couple into the differential lines, receiver chip 16xe2x80x3 can pass in and out of the power-down state, causing logical problems. Thus the high-impedance state is not as useful as the dual-high state for signaling power-down, causing a non-standard differential drive to be needed.
What is desired is a differential receiver that can detect a power-down condition on a differential pair of lines. It is desirable to use standard differential transmitters to signal a power-down condition to a modified differential receiver.